18.104.22.168 Non-sampling Errors
When a complete enumeration of units in the universe if made, one would expect that it would give rise to data free from errors. However, in practice it may not be so. For example, it is difficult to completely avoid errors of observation or ascertainment. So also in the processing of data, tabulation errors may be committed affecting the final results. Errors arising in this manner are termed non-sampling errors, as they are due to factors other than the inductive process of inferring about the population from a sample. Thus, the data obtained in an investigation by complete enumeration, although free from sampling error, would still be subject to non-sampling error, whereas the results of a sample survey would be subject to sampling error as well as non-sampling error.
Non-sampling errors can occur at every stage of planning and execution of the census or survey. Such errors can arise due to a number of causes such as defective methods of data collection and tabulation, faulty definition, incomplete coverage of the population or sample, etc. More specifically, non-sampling errors may arise from one or more of the following factors:
1. Data specification being inadequate and inconsistent with respect to the objective of the census or survey.
2. Inappropriate statistical unit.
3. Inaccurate or inappropriate methods of interview, observation or measurement with inadequate or ambiguous schedules, definitions or instructions.
4. Lack of trained and experienced investigators.
5. Lack of adequate inspection and supervision of primary staff.
6. Errors due to non-response, i.e., incomplete coverage in respect of units.
7. Errors in data processing operations such as coding, punching, verification, etc.
8. Errors committed during presentation and printing of tabulated results.
These sources are not exhaustive, but are given to indicate some of the possible sources of error. In a sample survey, non-sampling errors may also arise due to defective frame and faulty selection of sampling units.